KPI & Caster

Caster and King Pin Inclination (KPI) are used to define the location and angle of the steering axis (also known as the King Pin Axis) from the tire contact patch.

Caster & KPI Angle

Caster and KPI angle will cause the tire to camber as it is steered about the steering axis. The amount and direction that the tire will camber is given by the following formula.

∆ Camber = KPI (COS(Steering Angle)) + Caster (SIN(Steering Angle))

Because camber change is related to the Cosine of the steering angle the direction of the camber change will always have the same sign as the KPI angel on both inside and outside wheel. Since camber change is related to the Sine of the steering angle the direction of the camber change will be opposite on both the inside and outside wheels.

	Negative Caster	Positive Caster	Negative KPI	Positive KPI
Inside Wheel	Gains negative camber	Gains positive camber	Gains negative camber	Gains positive camber
Outside Wheel	Gains positive camber	Gains negative camber	Gains negative camber	Gains positive camber

Caster and KPI angle will vary the following parameters when the wheels are steered

·         Tire KPI Trail

·         Tire Caster Trail

·         Camber

·         Ride Heights

·         Roll Angle

·         Rake Angle

·         Corner Weights

·         Track Width (only of wheels being steered)

·         Wheelbase (Left and Right)

Mechanical Trail & Scrub Radius

Mechanical trail is the distance from where the steering axis intersects the ground to the tire contact patch when viewed from the side of the vehicle. While Scrub radius is the distance from where the steering axis intersects the ground to the tire contact patch when viewed from the front of the vehicle.

The location of where the steering axis intersects the ground will determine the forces in the steering linkages. If the intersection point is offset from the tire contact patch the lateral and longitudinal grip on the tire will create a torque at intersection point which will generate a force in the steering linkages. The point where the steering axis intersects the ground is defined by the Mechanical Trail and Scrub Radius.

The larger the mechanical trail and scrub radius the more effort that is required to steer the wheels. Too large a mechanical trail and scrub radius will cause heavy steering. While too small a mechanical trail and scrub radius will cause steering with little feel and feedback.